1. Field of the Invention
The present invention relates to an exhaust gas purifying apparatus for an internal combustion engine and, particularly, to an exhaust gas purifying apparatus including a NOx removing device and which performs a regeneration process for removing sulfur oxide (SOx) accumulated in the NOx removing device.
2. Description of the Related Art
Japanese Patent Laid-open No. 2003-120373 (JP '373) discloses an exhaust gas purifying apparatus having a NOx removing device. According to the disclosed apparatus, a regeneration process for removing sulfur oxide accumulated in the NOx removing device is performed. Specifically, a control which increases a fuel injection amount compared with the usual amount (i.e., a weak lean low-temperature combustion control), and a fuel addition control, which supplies fuel to the exhaust system, are performed until the temperature of the NOx removing device exceeds a predetermined temperature (e.g., 600 degrees Centigrade). When an air-fuel ratio detected by an air-fuel ratio sensor provided in the exhaust system becomes equal to or richer than the stoichiometric ratio, the fuel addition control is stopped and only a weak lean low-temperature combustion control is performed.
According to the apparatus disclosed in JP '373, a reducing component supply mechanism is required in order to supply fuel directly to the exhaust system. Therefore, it is preferable to supply reducing components to the exhaust system by a post injection (i.e., a fuel injection in the expansion stroke or the exhaust stroke after execution of the main injection). However, if an execution time period of the post injection becomes relatively long, dilution of the lubricating oil or degradation of the fuel consumption may easily result. Therefore, it is preferable that the execution time period of the post injection be controlled to be kept to a minimum.
On the other hand, it is necessary to raise a temperature of the NOx removing device to a very high temperature to remove (i.e., reduce) the accumulated sulfur oxide. However, it is confirmed that the reduction of sulfur oxide is possible at a temperature lower than 600 degrees Centigrade, e.g., at a temperature of 500 degrees Centigrade. Therefore, it is possible to remove sulfur oxide by increasing an injection amount of the usual fuel injection (by enrichment of the air-fuel ratio) without supplying fuel directly to the exhaust system or performing the post injection.